Baker's Cyst

Overview

A Baker’s cyst, also called a popliteal cyst, is a collection of synovial fluid at the back of the knee. It usually develops when excess fluid produced within the knee joint passes into the bursa between the semimembranosus tendon and the medial head of the gastrocnemius muscle.

In adults, a Baker’s cyst is commonly associated with an underlying knee disorder, particularly osteoarthritis, inflammatory arthritis or a meniscal tear. It may cause no symptoms or present as a lump, fullness and tightness behind the knee. Treatment focuses on symptoms and the underlying knee condition. Sudden calf pain and swelling may indicate cyst rupture but also require assessment for deep vein thrombosis (DVT).

Aetiology & Risk Factors

• Osteoarthritis: Degenerative joint change can increase synovial fluid production.
• Inflammatory arthritis: Conditions such as rheumatoid arthritis can cause persistent synovitis and effusion.
• Internal knee injury: Meniscal tears and other intra-articular injuries may lead to excess joint fluid.
• Recurrent or persistent knee effusion: Fluid can pass from the joint into the posterior bursa and accumulate there.
• In children, a Baker’s cyst may occur without an identifiable underlying knee disorder.

Clinical Manifestations

A Baker’s cyst may be found incidentally and cause no symptoms.

Unruptured Cyst

• A soft lump or sensation of fullness at the back of the knee, typically towards the medial side.
• Posterior knee tightness, discomfort or stiffness, particularly when the knee is fully extended.
• Knee pain, swelling or reduced movement related to the cyst or its underlying joint condition.
• A large cyst may cause lower-leg swelling or, rarely, symptoms from pressure on nearby nerves or blood vessels.

Ruptured Cyst

• Sudden pain and swelling behind the knee and in the calf.
• Calf tightness, redness or tenderness as fluid tracks down the leg.
• A presentation that can closely resemble DVT.

Diagnosis

The history and examination should assess both the posterior knee swelling and its likely underlying cause. Examine the knee for effusion, tenderness, restricted movement and signs of osteoarthritis, inflammatory arthritis or internal derangement. A typical cyst may feel firmer with the knee extended and softer when it is flexed.

• Ultrasound: Confirms that a popliteal swelling is fluid-filled and helps distinguish it from a solid mass. Venous duplex ultrasound is appropriate when DVT is suspected.
• Knee X-ray: Does not show the cyst itself but may identify osteoarthritis or other bony pathology.
• MRI: May be useful when the diagnosis is uncertain, the swelling is atypical, or a meniscal tear or other intra-articular lesion is suspected.

Consider other causes of a popliteal mass or calf swelling when the presentation is atypical. Acute calf swelling requires particular care because a ruptured cyst and DVT can produce similar symptoms.

Treatment

Asymptomatic or Mildly Symptomatic Cyst

• Reassure and observe if the cyst is small and not troublesome.
• Identify and manage the underlying knee disorder where possible.
• Modify activities that aggravate knee pain. Simple analgesia or an NSAID may be considered when appropriate.
• Physiotherapy can help restore knee movement and strengthen the surrounding muscles when indicated by the underlying condition.

Persistent Symptomatic Cyst

• Reassess the knee for ongoing effusion, synovitis or internal derangement.
• A corticosteroid injection into the knee joint may reduce inflammation when clinically appropriate.
• Ultrasound-guided aspiration may relieve a large or painful cyst, but fluid can reaccumulate if the underlying joint problem persists.
• Orthopaedic assessment may be appropriate for persistent symptoms despite non-operative care, repeated recurrence or significant pressure on nearby structures. Surgery is uncommon and usually addresses the underlying knee pathology as well as the cyst.

Suspected Rupture

• Assess promptly for DVT when calf pain or swelling is new or increasing.
• Once DVT and other urgent causes have been excluded, treatment is generally supportive, including rest, elevation and suitable analgesia while the fluid is reabsorbed.

Complications & Prognosis

Complications

• Rupture with acute calf pain and swelling that mimics DVT.
• Recurrence after aspiration or other treatment, especially if the underlying knee condition remains active.
• Rare compression of nearby nerves or blood vessels by a large cyst.

Prognosis

Many Baker’s cysts improve with observation or non-operative treatment, and some resolve without intervention. Symptoms and recurrence depend largely on the underlying knee disorder. Persistent or recurrent swelling should prompt reassessment of that disorder and confirmation that the diagnosis remains correct.

References

1. American Academy of Orthopaedic Surgeons. Baker’s Cyst (Popliteal Cyst). OrthoInfo. AAOS.
2. NHS. Baker’s cyst. NHS.
3. Ward EE, Jacobson JA, Fessell DP, Hayes CW, van Holsbeeck M. Sonographic detection of Baker’s cysts: comparison with MR imaging. AJR Am J Roentgenol. 2001. PubMed.